Heat exchange for the purpose of cooling air applies to many purposes, including air conditioning, space refrigeration, and dehumidification. A vapor-compression refrigeration system is conventionally used for these and other air cooling purposes. Vapor-compression refrigeration systems are well known, and are a principal technology used for condensing water from air, such as, for example in an atmospheric water harvesting unit. A typical vapor-compression refrigeration circuit includes a compressor, a condenser, an expansion valve, and an evaporator connected in series by a refrigerant piping. During operation, the compressor forces refrigerant from its outlet through the piping circuit sequentially through the condenser, the expansion valve, the evaporator, and back into the inlet of the compressor. Cold refrigerant is passed through the evaporator which cools air flowing across the evaporator by absorbing heat from the air as it passed across the evaporator.
The heat exchange between the cold refrigerant flowing in the evaporator and the air flowing across the evaporator is used in atmospheric water harvesting to extract water from the air by condensing water vapor dissolved in the air. The general principals and attributes of atmospheric water harvesting are well understood in the art. An exemplary atmospheric water harvesting device is disclosed in U.S. Pat. No. 7,954,335, the entirety of which is incorporated herein by reference.
While atmospheric water harvesting is understood in the art, a brief discussion of the general principals is with worth having herein. Generally, to condense water from air, a high surface area heat exchanger, such as the evaporator in a vapor-compression refrigeration system, is maintained at a temperature below the dew point of the air that is incident upon it. The moist air is passed through or over the chilled surfaces of the heat exchanger which further lowers the temperature of the air and condenses the water vapor dissolved in the air. The condensed water falls, by gravity, and is collected for use. Water condensation is well known as a byproduct of chilling air for other purposes, but water produced as a byproduct of chilling air for reasons other than water production is generally unsafe for drinking.
Atmospheric water harvesting generally produces high quality potable water from the air in the general vicinity of its place of use is pure and safe for immediate drinking with very little additional treatment required. Producing potable water near its place of use removes the requirement for either temporary or fixed water delivery systems such as pipelines or bulk water tankers or bottled water. Production of high-quality water at or near its place of use saves the energy that would otherwise be used for transport or to fabricate and maintain a water transport system. In addition, water harvesting produces virtually no waste products. Water harvesters are environmentally beneficial, especially on islands or in remote locations because building and maintenance of water delivery systems are not required and the waste attributed to used water bottles is not an issue. The water may be stored and treated against bacterial and other contamination using relatively inexpensive, simple systems because it is essentially pure, distilled water to begin with.
When significantly chilling air or removing a maximum amount of water from air through condensation, energy efficiency is usually low. The energy efficiency of an atmospheric water harvesting unit is based on many factors, including, but not limited to the refrigeration capacity of the vapor-compression refrigeration system, the relativity humidity of the air from which water is being harvested, and the evaporator construction.
Condensation on the evaporator takes place by reducing the temperature of the humid air to the point at which it is depressed below dew point. Where intake air is at a high humidity, for instance in excess of 85% relative humidity (RH), water will begin to condense with relatively little energy consumed by chilling of the air itself. The delivery of air to the evaporator at approximately 90+% RH is the primary objective for the most economic water production through condensation. The sensible heat of the humid air, which is the term applied to heat associated with temperature change, first must be removed to lower the temperature of the air in order to bring the air to as near 100% RH as possible, at which point the air is supersaturated and further cooling initiates condensation. As the temperature of the humid air falls further, condensation proceeds as the latent heat, which is that required to cause the water vapor to condense to liquid water, is removed by heat exchange. Following the initiation of condensation, water is produced and can be extracted by removing both sensible heat and latent heat from the humid air, which remains at supersaturation as it cools further.
In order to achieve optimum energy efficiency, it is desirable to minimize the degree of sensible heat removal to increase the refrigeration potential that is available for latent heat removal. In other words, it is desirable to increase the latent heat to sensible heat removal ratio. If the evaporator is operated at very low temperatures, there is a higher energy cost because increasing sensible heat must be removed along with latent heat. The refrigerant compressor is the primary energy cost in a vapor compression refrigeration system. Although very high chilling potential has the potential to remove proportionally more water from the air, as would be desirable in a dehumidifier in which drying the air is the objective, as temperature is decreased the energy cost per volume of water is increased. It has been found that it is more energy efficient to move higher volumes of air and remove only a relatively small amount from it at lower chilling potential than to greatly chill the evaporator. Some combination of minimum chilling potential and airflow will produce the maximum energy efficiency as it reduces load on the compressor.